When electric motors are used within three-phase power distribution systems, the motor can become over-heated upon the occurrence of a condition commonly known as "single-phasing". The single-phasing occurs when a fuse in one phase operates to isolate the phase or the wiring within one of the phases becomes other-wise disconnected. The remaining two phases continue to feed the motor causing the motor to overheat. Although the circuit current increases during the overheating conditions, most circuit interruption devices are not sufficiently sensitive to respond to the increased current resulting in eventual damage to the motor.
Most devices employed to interrupt circuit current upon the occasion of a single-phasing condition require separate sensing devices to measure the voltage across a set of fuses or to measure the current imbalance within the motor itself.
U.S. Pat. No. 4,060,843 discloses separate sensing means coupled to three-phase power lines for determining the occurrence of single-phasing. U.S. Pat. No. 4,021,703 describes the use of a DC ripple detecting device capable of being utilized on a three-phase circuit to detect phase imbalance. U.S. Pat. No. 4,837,654 teaches the use of signal amplifying transistors connected in a serial amplification configuration to detect single-phasing.
One purpose of the instant invention is to describe a circuit capable of detecting both phase loss and phase imbalance without requiring separate sensing devices apart from the current transformers used within electronic overcurrent circuit interruption devices.